Slot stigmator



Dem- 1', 1970 J. H. REISNER SLOT STIGMATOR Filed April .27, 1966 I INVENTOR. .jN/U H, Ensusg BY 40mg /151; I

n'r'rozugy United States Patent 3,544,836 SLOT STIGMATOR John H. Reisner, Haddonfield, N.J., assignor, by mesne assignments, to Forgflo Corporation, Sunbury, Pa., a corporation of Delaware Filed Apr. 27, 1966, Ser. No. 545,681 Int. Cl. H01j 29/56 US. Cl. 315-31 5 Claims ABSTRACT OF THE DISCLOSURE Disk means are disclosed to overcome the defocusing effect of a non-symmetrical magnetic electron lens in an electron microscope. The disk means is thin enough to fit between the pole pieces of the magnetic lens and still leave room for the specimen to be inserted between the pole pieces of the magnetic lens. The disk means includes a plurality of disks so constructed and arranged as to produce a plurality of different and distinct electron lenses.

This invention relates to electron apparatus and more particularly to lens compensating means for electron beam apparatus such as an electron microscope.

In an electron optical apparatus such as an electron microscope, a beam of electrons is formed by an electron gun. The electrons are projected through a specimen to be examined and then through a magnetic electron lens which focuses an image of the object on a viewing plate. If the magnetic field provided by the magnetic lens is not symmetrical about its central axis, the eifect on the beam will be to cause distortion of the image. Asymmetry in the magnetic field is usually caused by anisotropy of the magnetic material from which the pole pieces comprising the magnetic lens are made, and by mechanical imperfections in the construction and adjustment of the lens. The asymmetry causes a type of image distortion that is called anisotropic astigmatism. Means for correcting field asymmetry, or stigmators, are known. However, the known stigmators are relatively long, about 0.15 of an inch, in the direction of the electron beam. In working with high energy electron lenses whose focal lengths are about 0.083 of an inch, where for proper focusing the specimen must be inserted between the pole pieces of the lens, a known stigmator cannot be successfully fitted into a lens of such short focal lengths since the distance between the pole pieces of such a lens does not allow space for both the specimen and a known stigmator. If the stigmator is positoned outside of the pole pieces along the electron beam, more voltage must be applied to stigmator to cause it to correct for anisotropic astigmatism.

It is an object of this invention to provide an improved anisotropic astigmatism correction means or stigmator.

It is another object of this invention to provide a stigmator which is short in the direction of the beam of electrons that pass therethrough compared to known stigmators.

It is still another object of this invention to provide a stigmator which is sufficiently short in the direction of the electron beam that passes trerethrough to be mounted in a short focal length magnetic lens.

In accordance with this invention, a plurality of fiat conductive disks, each having a hole therethrough, are provided. The disks are so stack that the flat sides of each disk, except the outside ones, are separated only by insulation from the flat sides of other disks and the centers of the holes throught he disks are aligned. The holes through the outside disks are round and the holes through the intermediate disks are elongated and are symmetrical with respect to their respective centers whereby the holes 3,544,836 Patented Dec. 1, 1970 in the intermediate disks each have a long and a short axis perpendicular to each other. The intermediate disks in the stack are so arranged that hte long axes of the holes are at an angle with each other. The stack of disks may be placed between the pole pieces of an electron lens in such a position that the electron beam which has passed through the specimen also passes through the holes in the stack of disks. Ground or a reference potential is applied to the outside disks and adjustable voltages are applied to the inner disks with respect to the outside disks. The voltage applied to the inner disks may be adjusted to correct for anisotropic astigmatism of the beam whereby the stack of disks so stacked and so energized acts as a stigmator.

The invention may be better undestood from reading the following description in connection with the accompanying drawing in which:

FIG. 1 is a diagrammatic showing of a stigmator in accordance with this invention in place in a magnetic electron lens,

FIG. 2 is a plan view of the individual disks making up the stigmator of FIG. 1, and

FIG. 3 is a circuit diagram of a means for energizing the stigmator of FIG. 1, showing the relative angular and spacial positions of the conductive disks comprising the stigmator.

Turning first to FIG. 1, portions of the upper and lower magnetic pole pieces 10 and 12, as viewed in this figure, are shown in sections. As understood, the pole pieces 1.0 and 12 are symmetrical about the electron beam 14 which goes down along the axis thereof. A cup-like specimen holder 16 is also arranged symmetrically with respect to the beam 14. The specimen holder 16 is supported on the upper pole piece 10 by conventional means not shown. A hole in the bottom of the specimen holder 16 is closed by a finely meshed screen 18 on which the specimen, not shown, may be laid. The specimen holder 16 and the screen 18 are both so positioned that the specimen will be positioned between the pole pieces 10 and 12. An aperture means 20, which may be supported by the lower pole piece 12, is provided, the aperture through the top of the aperture means 20 being at about the level of the upper surface of the lower pole piece 12. The electron beam 14 is produced by an electron gun, not shown, and passes downward through the pole pieces 10 and 12. After the electrons comprising the beam pass through the specimen and the objective lens comprising the pole pieces 10 and 12 and the aperture means 20, which intercepts electron beams that are scattered by the specimen or by the screen 18, the beam is focused on a viewing plate (not shown) to give a magnified image of the specimen. However, as noted above, due among other things, to anisotropy of the magnetic material from which the pole pieces 10 and 12 are made and due to mechanical misadjustments, anisotropic astigmatism of the image occurs. This astigmatism is minimized or cured by the stigmator to be described.

In passing through the lens comprising the pole pieces 10 and 12, the beam is rotated in a helical manner through an angle of about 120. The stigmator should operate to correct the astigmatism within a distance during which the beam is rotated less than because otherwise, the corrections made by the stigmator would be averaged out along all the directions perpendicular to the direction of travel of the beam, whereby the stigmator would not correct the astigmatism. Optimum operation of the stigmator would result if its length were zero whereby all the correction would be applied to the beam before it had a chance to rotate, however, this is mechanically impossible. To be useful for its purpose, a stigmator must be provided whose length in the direction of travel of the beam is not more than the distance through which the beam rotates about 60. When the distance between the lower edge of the upper pole piece and the upper edge of the lower pole piece 12 is only about 0.41 of an inch, as in the case of a short focal length magnetic lens, and when the maximum rotation of the beam while passing through the sigmator is 60, it is seen that the thickness of the stigmator to be described cannot exceed about 0.2 of an inch. However, when the specimen is also positioned between the pole pieces 10 and 12, as is necessary, the space left between the pole pieces for the stigmator may not exceed about 0.08 of an inch. A stigmator has been built having a thickness of less than about 0.033 of an inch, the stigmator including nine disks such as those illustrated in FIG. 2.

The disk 22 shown in plan view at the top of FIG. 2 is an end disk of which two are required. This disk 22 is made of a conductive non-magnetic material and is about A, of an inch thick and has a round hole 24 through the center thereof having a diameter of about of an inch. An edge of the disk 22 is cut away at 26 to provide clearance for a tab 28 on another conductive disk, as will be explained.

Four circular disks 32 of insulating material about 31, of an inch thick and having a hole 34 therethrough having a diameter of about $5 of an inch are provided. These insulating disks will be placed between stacked conducting disks as will be explained.

A further conductive circular disk 30 is provided having a thickness of about 5, of an inch. A slot-like hole 42 having rounded ends is provided through the disk 30. The longer dimension or axis of the hole 42, which is not critical in length, is at least two and a half times as great as its smaller dimension or axis, the two axes being perpendicular to each other. The smaller dimension of the hole 42 is at least as great as the diameter of the hole 24. The diameter of the hole 34 is at least as great as the larger dimension of the hole 42. A tab 28 is electrically connected to the disk 30, as by welding, in a position such that the extended shorter axis of the hole 42 passes through the tab 28.

Two further disks 36 and 38 are provided. The disks 36 and 38 are identical and they difier from the disk 30 only in that a portion of the disk 36 is cut away as at 40 through about a 60 arc starting at the left, as viewed in FIG. 2, at the line through the larger axis of the hole 42 therethrough, the cut away portion extending downward in a counterclockwise direction. The disk 38 is merely the disk 36 rotated 180 about an axis through the smaller axis of the hole 42. The stigmator which comprises the disks of FIG. 2 is assembled and positioned between the pole pieces and 12 in FIG. 1 as will be described.

Turning to FIG. 1, a non-magnetic conductive cylinder spacer 44, which is internally threaded along a portion of its length is provided between the pole pieces 10 and 12 and is coaxially arranged with respect thereto. The bottom part of the spacer 44 is partially closed by a closure member 46 having a hole therethrough. A conical flange 48 extends around the hole in the member 46 and projects downward and outward to contact the inner periphery of the lower pole piece 12. The member 46 and the flange 48 may be cut away as at 50 for ease of assembly of the spacer with the pole piece 12, if desired. The upper surface of the closure 46 acts as a support for the lower outside disk of the stigmator as will be explained. A ringlike spacer 52 of insulating material, having three radial holes 53 therethrough, only one of which is shown, is provided within the cylinder 44. The stigmator disks are mounted on the closure member 46 within the spacer 52, as will be explained, and a conductive ring-like top clamp plate 54, which is threaded on its external cylindrical periphery, is screwed down on the top disk of the assembled stigmator.

In FIG. 3, the conductive disks or electrodes 22, 36, 30,

38 and 22 are shown in their relative spacial and also in their relative angular positions when assembled (as explained below) between the pole pieces 10 and 12 of a magnetic electron lens, the insulating disks 32 (one of which appears between each two conductive disks) being omitted for clarity. The arrow 66 in FIG. 3 represents the electron beam of the microscope in which the described stigmator is to be mounted. For clarity of illustration, the disks of FIG. 3 are shown as being widely separated but they are assembled as close together as they can be placed and still be insulated from each other.

The stigmator is assembled by laying an end disk 22 (the left one as viewed in FIG. 3) on the support member 46. Then an insulator disk 32 is laid on the disk 22. Next, the disk 36 is laid on the disk 32 in such a manner that the tab 28 of the disk 36 is above the cut away portion 26 of the disk 22, the tab 28 extending through a hole 53 in the insulator ring 52. The next step is to lay an insulator disk 32 on the disk 36. Then lay the disk 30 on the disk 32 with the tab 28 of the disk 30 over the cut away portion 40 or the disk 36 and with the tab 28 extending through another hole 53 in the insulator 52. Then an additional insulator disk 32 is laid on the disk 30. The disk 38 is then laid on this disk 32 with the tab 28 of the disk 38 extending in such a manner that the tab 28 of the disk 30 is under the cut away portion of the disk 38, the tab 28 of the disk 38 extending through the third hole 53 in the insulator 52. An additional insulator disk 32 is laid on the disk 38 and a second end 'plate 22 is laid on the disk 38 in such a manner that the cut away portion of the disk 22 is above the tab 28 of the disk 38. All the disks are coaxially positioned. Therefore, a thin stigmator having five conductive electrodes or disks that are insulated from each other is provided. The long axes of the holes 42 through the disks 30, 36 and 38 are at 60 angles with each other. The distance between the end plates 22, 22 is about equal to the smaller axis of the holes 42 through the disks 30, 36 and 38, whereby each disk 30, 36 and 38 acts with the end disks 22 as an electron lens, three electron lenses thereby being provided by the assembled stigmator. The distance between any two alternate ones of the conductive disks 22, 30, 36, 38, or 22 however, is smaller than the small axis of the hole 42 in an intermediate conductive disk whereby three adjacent conductive disks do not act as an electron lens.

Energization of the disks comprising the stigmator is accomplished as follows: The clamping of the disks comprising the stigmator together by the clamping plate 54 connects the upper and the lower disks 22 to the pole pieces 10 and 12 which are kept at a constant reference potential. The tabs 28 on the disks 30, 36 and 38 extend through respective holes 53 in the insulator 52 and are accessible through registering holes in the cylindrical spacer 44, only one hole through the spacer 44 being shown in FIG. 1. The disks 30, 36 and 38 are energized by applying direct voltages thereto with respect to ground through conductors (see FIG. 3, not shown in FIG. 1) connected to the respective tabs 28.

The tab 28 on a disk 36 is connected to a point 70 on a circular potentiometer resistor 68. The tab 28 on the disk 30 is connected to a point 72 on the resistor 68, away from the point 70, and the tab on the disk 38 is connected to a point 74 which is 120 away from both of the points 70 and 72. A radially positioned rotating contactor 76 rotates about the center 78 of the resistor 68 and the contactor 76 is connected to ground or reference potential by a conductor 80. Another radially positioned rotating contactor 82, which is held in line with the contactor 76 and is insulated therefrom, is connected to a slider 84 on a potentiometer 86. The potentiometer 86 is connected across a voltage source (not shown) which may be of either polarity with respect to ground or to a reference potential. By rotating the contacts 76 and 82 as a unit, the voltages, on the disks 30, 36 and 38 may be varied with respect to each other and with respect to ground. In this manner, energization of the three electrostatic lenses comprising each one of the intermediate disks 36, 30 and 38 taken individually with the two end disks 22, is adjusted to apply deflecting voltages to the beam 66 in a direction perpendicular to the direction of the beam and at 60 with each other and in such a manner as to minimize or cure the anisotropic astigmatism of the beam passing therethrough.

As noted above, in the described stigmator, the individual slotted disk electrodes 30, 36 and 38 act almost independently of the other slotted electrodes. That is, each of the slotted electrodes 30, 36 and 38, and the end elctrodes 22, 22 complete a different and distinct lens. This is due to the fact that the long dimensions or axes of the slots 42 in the slotted disks 30, 36 and 38 are more than two and a half times the short dimensions or axes thereof, whereby the several lenses comprising the disks 30, 36 and 38 are almost completely cylindrical and whereby the several lenses do not shield each other in the plane of action thereof, and due to the fact the separation of the slotted electrodes 30, 36 and 38 is small compared to the small dimension of the slot 42. For example the separation of the slotted electrodes 30, 36 and 38 is about one-fifth of the small dimension of the slot 42. The described stigmator provides a plurality (three in the embodiment described) of separate cylindrical lenses, which are axially close together compared with the focal lengths in which the efiect of the several lenses is vectorially addable without substantial interaction between the several lenses. Such action of the stigmator is necessary to correct for anisotropic astigmatism.

While the stigmator is described as being mounted in a magnetic lens, it may be mounted at any desired location along an electron beam.

Many modifications of the stigmator described will be evident to a person skilled in the art. For example, instead of providing insulating disks 32, each of the intermediate disks 30, 36 and 38 may be coated with insulation over the contacting surfaces thereof. Other methods of clamping the disks comprising the stigmator are evident such as the provision of an upper and a lower clamping ring held together by bolts extending therebetween which are insulated from the intermediate disks 30, 36 and 38. With such a clamping means, an insulating spacer 52 may not be necessary. Also, the stigmator may be used in cathode ray apparatus other than microscopes that do or do not include magnetic lenses. Therefore, the above illustration is to be considered as illustrative and not in a limiting sense.

What is claimed is:

1. The combination of a short focal length magnetic 6 lens and a stigmator, said stigmator being arranged between the pole pieces of said lens, said stigmator comprising a stack of at least four conductive disks,

said conductive disks each having a hole therethrough,

the holes through the end disks of said stack of disks being round and the holes through the intermediate disks of said stack of disks each having a long and a short axis, said long axis being at least two and one-half times said short axis,

the centers of the holes through the disks being aligned and the long axes of the holes in the intermediate disks being at an angle with each other, the distance between each two alternate disks being less than the short axis of the holes in the intermediate disks, and

means for insulating said disks from each other,

said holes being positioned to pass the electron beam of said magnetic lens.

2. The combination of claim 1, said stigmator comprising three intermediate disks, the angles between the long axes of the holes in the intermediate disks being substantially 3. The combination of claim 1, said stigmator comprising three intermediate disks, the angle between the long axes of the holes in the intermediate disks being substantially 60,

the distance between said end disks being substantially equal to the small axis of the holes through the intermediate disks.

4. A combination of claim 1 including means for electrically connecting said end disks to said pole pieces, and

means to apply voltages to said intermediate disks with respect to said end disks.

5. The invention as expressed in claim 4 in which there are three intermediate disks and in which the angle between the long axes of the holes through the intermediate disks is substantially 60 with respect to each other.

References Cited UNITED STATES PATENTS 2,884,559 4/1959 Copper et a1. 315-31TV RICHARD A. FARLEY, Primary Examiner I. G. BAXTER, Assistant Examiner U.S. Cl. X.R. 

